This invention relates to the comminution of solid materials and, more particularly, is concerned with an apparatus and a process for the attrition-grinding of particulate solids.
The comminution of particulate solids by attrition-grinding grinding has been well known since about 1948 and essentially comprises the agitation of a mixure comprising a slurry of the particulate solid to be comminuted and a granular grinding medium. This method of comminuting solids is generally used when it is desired to reduce a particulate solid to particles of very small size, for example to particles substantially all of which are smaller than 53 microns (i.e. they will pass through a No. 300 mesh B.S. sieve).
Several designs of attrition-grinding mill for carrying out an attrition-grinding process have been suggested, but at the present time the attrition-grinding mills used on a commercial scale are generally constructed so that a feed slurry of the particulate solid to be comminuted can be fed as a continuous stream to the mill at the bottom of an upright grinding chamber and the ground product slurry can overflow continuously through a sieve at the top of the grinding chamber, the sieve having apertures of a size such that the granular grinding medium is retained in the grinding chamber. It may be noted here that the particulate solid in the feed slurry will consist predominantly of particles substantially smaller than the apertures in the sieve, and that the purpose of the sieve is to retain the grinding medium in the grinding chamber and not to determine the particle size of the solids in the ground product slurry.
When the product is to be transported as a slurry it is desirable to work with the feed slurry at as high a solids content as possible, but it is found that if a feed slurry of high solids content is subjected to prolonged grinding in an attrition-grinding mill constructed as described above, considerable heat is generated and the sieve tends to become blinded through the evaporation of the liquid of the slurry and the deposition of solids in the apertures of the sieve. It is also found that there is a tendency for the concentration of the granular grinding medium to be highest in the upper part of the grinding chamber which means that the ratio of granular grinding medium to feed slurry varies over the height of the grinding chamber; this may by disadvantageous in some types of attrition-grinding mill. A further disadvantage is that with an attrition-grinding mill in which the ground product slurry continuously leaves the grinding chamber there is a tendency for the flow of ground product slurry through a sieve to decrease steadily as the grinding process proceeds since the slurry carries particles of the granular grinding medium with it and causes them to lodge in the apertures of the sieve thus restricting the flow. Since the ratio of granular grinding medium to slurry is important in determining the optimum grinding efficiency, any variation in the flow of ground product slurry from the grinding chamber, without a corresponding change in the flow of feed slurry to the grinding chamber, may eventually have an adverse effect on the efficiency of the process.